Remove print from test
Clean target
Restructure test and example files

 - Upgrade to testthat 0.8
 - Use new preferred structure for testthat
 - Only put interesting files in inst/extdata
 - Put test data under tests/data

Note that validation tests are now broken
Embed validation tests in test_that setup
Explain testing infrastructure in README
Clear error when variance decomposition fails
Make a start with regression tests
Checks in functions that assume consistency

 - forest.mtc.result
 - relative.effect
 - rank.probability
Add binom/cloglog survival regression test